Smart cards may have stored a unique identification number (UID), with which they can be unambiguously identified. A reader can—in particular if there are a number of smart cards within the radio range of the reader select a certain smart card and communicate to this selected smart card. This identification can be fixed so that the card is always unambiguously identified. Alternatively, this identification can be random so that the card can be tmambiguously identified during a session (a session takes as long until the card leaves the radio range of the reader again).
A random identification may be chosen if the privacy and/or traceability of its owner shall be guaranteed because a fixed identification can unambiguously be associated with an individual, whose location then can easily be determined and tracked as well. In most eases this is undesired for card owners.